Compression and tension are forces that cause a material to be pressed together or pulled apart. You compress a chair when you sit on it. Squeezing a sponge is a compression force. Can you think of others? We think of tension in a rubber band as it stretches. But just similar to compression in a chair that you don’t see when you sit on it, you don’t see compression in the chair or tension in the cables holding hanging lights or plants from the ceiling. Two of the common forces that building components experience are compression and tension. When properly designed, a bridge or building will be built to withstand any potential load that the structure may experience.
Sturdy Cardboard Box
Semi-crushed Cardboard Box
Take an empty soda can, and stand on it. Balance on one foot. Does the can compress? Now make a slight dent in the can by pressing your thumbs into the middle. Are you able to crush the can with one foot now? Can you stand on the can? Before the can has a dent in the middle, it can withstand much higher compressive loads. This means that you can probably stand on the can when it is still perfectly straight. Once the can has a kink or fold on its side, it is much easier to crush, or compress.
Repeat this activity with a brick. Obviously, you can’t bend the brick in the middle the way you may bend the can. Why do you think this is? Think about the differences in the materials and ways a brick is made as compared to an empty tin can.
Now try this activity using an empty cardboard box. Try this with different boxes that have different side thicknesses. A box with sturdy sides can support much more weight than a box with flimsy sides. Think about how the force of your weight transfers through the box to the ground below you. In order for the box to stay straight, it must support all of your weight. You may think of your weight passing “through” the box as unseen arrows pointing towards the ground. These invisible arrows represent the compressive force the box is carrying due to your weight on its top.
Watch the Compression and Tension video to help better understand how the forces of compression and tension affect construction materials.
Compression & Tension
Next Generation Science Education Standards
Scientific and Engineering Practices:
-Asking questions (for science) and defining problems (for engineering)
-Planning and carrying out investigations
-Cause and effect
-Structure and function
1. Discuss with a friend how buildings and bridges have to be designed to resist the forces of compression and tension. What effect would compression and tension forces have on bridges or buildings?
2. Draw force arrows downward to show how your weight is distributed on both the sturdy and the semi-crushed cardboard boxes. Draw larger arrows where you think the greater forces are. Are the arrows larger on the crushed box than they are on the crushed box? If they are larger, why did you choose to draw them this way?
3. Try stretching a non-deformed, empty soda can. Do you think the soda can withstands more tension force than it does compression force? Why do think this may be?